This invention relates to a system for controlling a hydraulic actuator such as implement cylinders or the like for use in construction vehicles like bulldozers.
As for the control device of the kind specified, there has heretofore been employed a device which is arranged to supply the fluid under pressure deliverred by a hydraulic pump driven by an engine through a control valve to an implement cylinder and a drain system.
However, such construction is disadvantageous in that when inching control of the implement cylinder is made most of the fluid under pressure deliverred by the hydraulic pump is throttled by a control valve or the flow rate of the fluid is controlled therein so that a major part of the power developped by the pump is consumed uselessly thereby generating a power loss.
In other words, at the time of full stroke operation control, all the fluid under pressure deliverred by the pump is effectively used, whilst at the time of inching control, only part of the deliverred fluid under pressure is effectively used and the rest of it is throttled causing a power loss.
Thus, the conventional control device suffers from such a disadvantage that, because a hydraulic pump having a delivery capacity to meet the flow rate of fluid at the time of full stroke operation control is used, a major part of the fluid under pressure is throttled or consumed ineffectively during inching operations thereby to cause a power loss. cl SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a control system for a hydraulic actuator which is capable of overcoming the above noted problems. Another object of the present invention is to provide a control system for a hydraulic actuator wherein large volume of hydraulic oil can be supplied to the actuator during full stroke operations while small volume of hydraulic oil can be supplied to the actuator during inching operations thereby reducing power loss at the inching operations.
A further object of the present invention is to provide a control system for a hydraulic actuator which is capable of improving response time or responsibility of the actuator during inching operations. In accordance with an aspect of the present invention, there is provided a control system for a hydraulic actuator comprising: an engine; a first hydraulic pump with large displacement capacity driven by said engine; a second hydraulic pump with small displacement capacity driven by said engine; a pilot pump driven by said engine; first pilot operated control valve means connected at the input side thereof to said first pump, the output side thereof being selectively connected to said hydraulic actuator and drain; second pilot operated control valve means connected at the input side thereof to said second pump, the output side thereof being selectively connected to said hydraulic actuator and drain; and third control valve means for pilot pressure connected at the input side thereof to said pilot pump, the output side thereof being connected to said first and second pilot operated control valve means wherein said second pilot operated control valve means is adapted to be actuated by a pilot pressure lower than a pilot pressure which actuates said first pilot operated control valve means whereby said hydraulic actuator is operated by hydraulic fluid from said second pump during inching operations of said third control valve means for pilot pressure and is operated by hydraulic fluid from both said first and second pumps when said third control valve means for pilot pressure is fully operated.
The above and other objects, features and advantages of the present invention will be readily apparent from the following description taken in conjunction with the accompanying drawings.